1. Field of the Invention
The present invention relates to a fuel pressure control device for an internal engine in which even if an abnormal condition occurs in a fuel pressure sensor in a high pressure fuel supply system, desired high fuel pressure control is continuously carried out by calculating an appropriate estimated fuel pressure based on an air-fuel ratio condition to achieve a reliable limp home mode (safe mode) operation.
2. Description of the Related Art
In one proposed conventional fuel pressure control device for an internal combustion engine, a high pressure fuel system including a high pressure pump for pressurizing a fuel at a high pressure and a fuel pressure sensor, and an ECU for performing feedback control such that the fuel pressure in a pressure accumulation chamber matches a target fuel pressure are provided. As fuel pressure control in the fuel pressure control device, if an abnormal condition occurs in the high pressure fuel system, a fuel pressure control valve is forcibly controlled such that the fuel pressure is adjusted to the maximum pressure (corresponding to the valve opening pressure of a relief valve as described later) for performing the safe mode operation (see JP 10-176587 A, for example).
Further, in another proposed conventional device, the fuel pressure control valve is forcibly controlled such that the fuel pressure in the pressure accumulation chamber is adjusted to the minimum pressure (corresponding to the adjustment pressure of a low pressure regulator as described later) for performing a safe mode operation (e.g., see JP 3,233,122 B).
In any of the above-mentioned conventional devices, when the fuel pressure sensor is in a normal condition, the drive pulse width of a fuel injection valve is corrected based on the fuel pressure detected by the fuel pressure sensor. Even if the fuel pressure changes, a predetermined amount of fuel is supplied to the internal combustion engine by injection.
However, when a failure occurs in the fuel pressure sensor, the correction of the fuel injection amount is not properly carried out. Therefore, an accidental fire or an engine stop may occur due to deviation of the air-fuel ratio. In order to avoid the circumstances, when a failure occurs in the fuel pressure sensor, the fuel pressure control valve is forcibly controlled such that the fuel pressure is adjusted to a known pressure, and the known pressure is used as an estimated fuel pressure to correct the drive pulse width for performing the safe mode operation.
That is, in the conventional device disclosed in JP 10-176587 A, when the abnormal condition of the sensor is detected, the fuel pressure feedback control is stopped, and the fuel pressure control valve is forcibly controlled such that the fuel pressure is adjusted to the maximum pressure. In the conventional device disclosed in JP 3,233,122 B, the fuel pressure feedback control is stopped, and the fuel pressure control valve is forcibly controlled such that the fuel pressure is adjusted to the minimum pressure.
The fuel injection amount corresponding to the drive pulse width of the fuel injection valve has nominal characteristics. In order to achieve the constant fuel amount at different fuel pressures, it is necessary to change the drive pulse width in correspondence with the fuel pressure. As the fuel pressure gets lower, the drive pulse width gets longer.
In consideration of the fuel injection at the minimum pressure, when an air intake valve is closed, since the compression pressure in a combustion chamber increases to a pressure equal to or greater than the minimum pressure, the fuel injection cannot be carried out. Further, since the injected fuel is released from an exhaust valve in the discharging process, expansion of the drive pulse width is constrained.
Therefore, in the case where the safe mode operation is carried out at the minimum pressure as disclosed in JP 3,233,122 B, the operation range of the safe mode operation is significantly limited due to the constraint.
Further, the injection amount corresponding to the drive pulse width varies depending on the individual difference between cylinders. Therefore, as disclosed in JP 10-176587 A, if the safe mode operation is carried out at the maximum pressure (>the fuel pressure by the normal feedback control), when the operation is carried out at a low load (idling operation, or deceleration operation), deviation between the injection amounts of the cylinders is large, and the reliable safe mode driving may not be achieved due to an accidental fire or an engine stop.
Moreover, in a period after switching to the forcible control for achieving the known pressure (the maximum pressure or the minimum pressure) until the actual fuel pressure is guided to the known pressure, the drive pulse width may be corrected erroneously due to the deviation amount between the known pressure and the actual fuel pressure, and an accidental fire or an engine stop may occur.
For example, in the case of JP 10-176587 A, in the period when the fuel pressure deviation occurs, i.e., in the period after the control is switched to the forcible control for achieving the maximum pressure until the actual fuel pressure is guided to the know pressure, since the maximum pressure is used as the estimated fuel pressure and the drive pulse width is erroneously corrected, the air-fuel ratio is deviated toward the lean condition to cause an accidental fire, and the rotational speed is lowered.
In the conventional fuel pressure control device for the internal combustion engine, in the case of JP 10-176587 A, when the abnormal condition occurs in the fuel pressure sensor, since the safe mode operation is carried out at the known maximum pressure, deviation in the injection amount at the time of low load operation is large, and thus, an accidental fire or an engine stop occurs. Therefore, it is not possible to carry out a reliable safe mode driving.
Also, in the case of JP 3,233,122 B, since the safe mode operation is carried out at the known minimum pressure, the operation range for the safe mode operation is significantly limited due to the constraint of the fuel injection period.
Furthermore, in the period after the control is switched to the forcible control for achieving the known pressure (the maximum pressure or the minimum pressure) until the actual fuel pressure is guided to the known pressure, deviation between the known pressure and the actual fuel pressure occurs. Thus, the drive pulse width is erroneously corrected, and an accidental fire or an engine stop occurs. Therefore, it is not possible to carry out a reliable safe mode driving.